In a conventional apparatus for detecting a track and focus errors in an optical head, a track error is detected, for instance, by one of a push-pull, heterodyne and three beams methods, while a focus error is detected, for instance, by one of a foucault and critical angle methods. Some of the methods are described in reports entitled "Ten billion bits on a disk" on pages 26 to 33 of "IEEE spectrum, August 1979, "Noise in a video disk system: experiments with an (AlGa)As laser" on pages 2007 to 2012 of "Applied Optics, Vol. 17, No. 13, 1 July 1978", "Video disk player optics" on pages 1993 to 2000 of "Applied Optics, Vol. 17, No. 13, 1 July 1978", and "Optical Readout of Video disk" on pages 304 to 308 of "IEEE Transaction on Consumer Electronics, November 1976". The details of these methods will be described later in more detail.
According to the conventional apparatus for detecting a track and focus errors in an optical head, however, there are disadvantages as follows.
(a) Track error
In the push pull method, at first, an object lens is actuated dependent on the amplitude and polarity of a track error signal to move perpendicularly to the light axis thereof by an actuator. In the occasion, the light axis of the object lens is liable to deviate from a dividing line of a two-divided optical detector so that light beam which is reflected from an optical disk is not exactly divided into two light beams to be equal in their light amounts of the dividing line. As a result, a direct current offset is induced in a track error signal so that a scope of controlling a track error becomes narrower. Such a disadvantage is also occured even in a case where an optical disk is inclined.
In the heterodyne method in which a track error signal is produced in accordance with a diffraction pattern formed at an edge of each pit on an optical disk, secondly, no track error signal can be detected from such a recording medium as having no pit, but pregrooves. Even more, an electric circuit becomes complicated because a heterodyne detection etc. are utilized therein.
In the three beams method in which a swing arm is adopted to forward a carriage, thirdly, a fluctuation of a signal intensity is occurred dependent on a rotating angle of the swing arm.
(b) Focus error
A focus error is detected in accordance with a focus error signal component of light beam including a RF and track error signal components which is reflected from an optical disk. For this reason, the other signal components, especially, the track error signal component is liable to be included in the focus error signal. As a result, the precision of controlling a focus error is decreased.